This invention relates to an electron image projector for projecting with substantially unity magnification a beam of electrons emitted by a cathode onto a target under the action of substantially uniform electric and magnetic fields, wherein the beam of electrons has a predetermined spatial pattern extending transversely to the electric field.
An electron image projector may be used in the manufacture of high resolution semiconductor devices for defining lithographically a pattern in an electron sensitive resist layer provided on a semiconductor wafer substrate. The patterned electron beam emitted by the cathode is projected onto the resist layer to define therein a pattern reproducing the pattern of the electron beam. After exposure the resist is developed and the patterned resist layer thus formed is used as a stencil during subsequent processing of the semiconductor wafer to form the device in question.
An electron image projector for entire wafer exposure and having the features mentioned in the opening paragraph is known, for example, from the paper by J. P. Scott entitled "1:1 Electron Image Projector" which appeared in Solid State Technology, May 1977, pages 43 to 47. The main advantages of this electron image projector can be summarized as fast exposure times and high resolution capability.
In the fabrication of integrated circuits, it is necessary to expose the semiconductor wafer to a respective pattern at several different lithographic stages. In order to ensure accurate registration of the patterns it is important to determine at each exposure the exact disposition of the wafer relative to the projected pattern. In the known 1:1 electron image projector the markers on the wafer are formed from tantalum oxide and are detected by the X-radiation they emit when irradiated by corresponding portions of the electron beam. Unfortunately, the efficiency of X-ray generation is low. Moreover, in order not to perturb the electric field the X-rays have to be detected on the side of the wafer remote from the cathode and hence they suffer significant attenuation before they are detected. In consequence the X-ray signal is very weak and the reference marker on the wafer has to be made relatively large to compensate. In order to have full alignment capabilities it is essential to have at least two reference markers spaced apart on the wafer. However in view of their large size and in order not to waste useful area of the semiconductor wafer it is in practice not feasible to employ more than two such markers on a 4 inch diameter wafer. With only two markers it is not possible to compensate for location distortions and it is therefore difficult to ensure good registration over the entire wafer. In other words the known 1:1 entire wafer electron image projector is not able to make full use of its high resolution capabilities because performance is limited by overlay errors.
A further disadvantage of the known 1:1 electron image projector is that the target, i.e. the resist coated wafer, constitutes an anode and in operation a large potential difference (typically 20 kV) is applied between the cathode and the anode to establish the electric field under the action of which the electron beam is projected from the cathode onto the target. This imposes stringent planarity requirements on the wafer in order not to distort unduly the electric field--and hence the electron trajectories--in the vicinity of the target. Inevitably the electric field will suffer some distortion at the periphery of the wafer. This can be a problem, especially if the wafer is held on the type of electrostatic chuck shown in FIG. 4 of the aforementioned paper by J. P. Scott, in which an overlapping lip is present at the surface of the wafer facing the cathode. In this case it is necessary to locate the wafer with great precision for each subsequent exposure in the image projector in order to obtain optimum registration.
A modified electron image projector which has the advantage that the dependence of the electron trajectories on the shape and/or disposition of the target can be reduced is disclosed in our co-pending (as yet unpublished) European Patent Application No. 84201263.5, corresponding to U.S. application Ser. No. 645,881, filed Aug. 30, 1984, now abandoned, and its continuation U.S. application Ser. No. 883,007, filed July 7, 1986. This modified electron image projector has a novel anode comprising a grid with an array of electron permeable regions, which array is at least as extensive as the spatial pattern of electrons emitted by the cathode. The grid is disposed between and parallel to the cathode and the target. In operation a substantially uniform electric field is produced between the cathode and the grid. Means are provided for producing a substantially uniform magnetic field parallel to the electric field to focus the patterned beam of electrons onto the target. Instead of an X-ray alignment facility, a backscattered electron detector is disposed generally between the grid and the target outside the region occupied by the projected electrons. The output of the detector is used to control the alignment of the projected beam with the wafer.
It is noted that backscattered electrons are much more readily detectable than X-rays used in a conventional electron image projector, particularly because the efficiency of their generation is some five orders of magnitude greater than that of X-rays.